SpaceX's Falcon 9 rocket has broken apart as it tried to land on a floating platform in the Pacific, marking the fourth such failure in the company's bid to recycle rockets.

However, the primary mission of the launch from Vandenberg Air Force Base in California went as planned, propelling into orbit a $180 million US-French satellite to study sea level rise.

"First stage on target at droneship but looks like hard landing; broke landing leg," the California-based company said on Twitter.

SpaceX said it lost contact with its live video link of the floating barge, or droneship, before the rocket came in for a landing, so no images were immediately available.

A commentator on SpaceX's live webcast said video would be made available in the coming hours.

"Unfortunately we are not standing upright on the drone ship at the moment but the good news here is that the primary mission is still on track," said the commentator, referring to the launch of the US-French oceans satellite, Jason-3.

SpaceX is trying to land its rocket back on Earth so it can re-use the parts in the future, as the company headed by Internet entrepreneur Elon Musk tries to make spaceflight cheaper and more sustainable than before.

However, no other company has attempted the ocean landing that SpaceX is trying to achieve.

The Jason-3 satellite, made by France and the United States, blasted off from Vandenberg Air Force Base in California, at 10:42 am (1842 GMT).

It aims to offer a more precise look at how global warming and sea level rise affect wind speeds and currents as close as one kilometer (0.6 miles) from shore, whereas past satellites were limited to about 10 kilometers (6.2 miles) from the coast.

The technology will monitor global sea surface heights, tropical cyclones and help support seasonal and coastal forecasts.

During a five-year mission, its data will also be used to aid fisheries management and research into human impacts on the world's oceans.

The satellite is the fruit of a four-way partnership between the National Oceanic and Atmospheric Administration (NOAA), the US space agency NASA, the French space agency CNES (Centre National d'Etudes Spatiales) and the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT).